Stroboscope control system

ABSTRACT

A system for triggering a strobe lamp at a low light intensity in response to signals which are automatically derived by frequency-division from much higher frequency synchronizing signals. It also includes a circuit which enables the production of a single, very high intensity strobe flash by one of the synchronizing signals in response to activation of associated photographic apparatus.

0 ijnited States Patent 1 1 1 1 3,746,899 Walker July 17, 1973 [54] STROBOSCOPE CONTROL SYSTEM 3,085,165 4/1963 Schaffert et a]. 307/273 3,181,009 41965 Fl h k 307 274 X [751 Invent! Gerald walke" Kmbemm 3,351,781 11/1967 J h iisz ii 307225 R [73] Assignee: Automation Industries, Inc., Los

Angeles, Calif. Primary Examiner-John Zazworsky [22] Filed: July 6, 1970 AttorneyDan R. Sadler [2]] Appl. No.: 60,986

Related US. Application Data [62] Division of Ser. No. 607,178, Jan. 4, i967, Pat. No. ABSTRACT A system for triggering a strobe lamp at a low light in- [52] US. Cl 30 /293, 307/2 307/252 J, tensity in response to signals which are automatically 3 derived by frequency-division from much higher fre- [51] Int. Cl. H03k 5/13 quency synchronizing signals. It also includesva circuit [58] Field of Search 307/273, 293, 225 R, which enables the production ofa single, very high in- 252 274 tensity strobe flash by one of the synchronizing signals in response to activation of associated photographic [56] References Cited apparatus.

UNITED STATES PATENTS 3,283,255 11/1966 Cogar 307/235 X 7 Claims, 8 Drawing Figures POWER LOW mnws/rr upp y clans/Iva CIRCUIT /4 may INTENSITY CHARGING CIRCUIT ION/ZINC;

VOLTAGE cmcu/r 02% f HIGH f FREQUENCY m/nws/rr urn/v4; o/wama cmcu/r rmmva cmcu/r snvc.

FROM CA HERA Patented July 17, 1973 4 Sheets-Sheet i Patented July 17, 1973 r 3,746,890

4 Sheets-Sheet I FROM v I 4 Sheets-Sheet 4 'llll-Illlllll lllllll l STROBOSCOPE CONTROL SYSTEM CROSS REFERENCE TO RELATED INVENTION This is a divisional application of copending application, Ser. No. 607,178, now U.S. Pat. No. 3,576,468 filed on behalf of GERALD E. WALKER, assigned to the same assignee as this invention.

BACKGROUND OF THE INVENTION This invention relates to a system for controlling the flashing of a stroboscope lamp continuously or for single-flash photographic purposes.

Known stroboscope control systems include systems for dividing the frequency of the synchronizing signals to a rate within the operating range of the stroboscope lamp. However, they require manual setting of the system to accommodate the diverse range of frequencies within which the synchronizing signals fall. Moreover, there is no known stroboscope control system which includes a circuit that is especially adapted to produce a single synchronized high intensity flash for photographic purposes yet protects associated switches from harm due to passage ofa very large current through the lamp.

SUMMARY OF THE INVENTION A circuit is provided to which external (or internal) synchronizing signals are supplied. This circuit automatically divides the frequency of these input signals to a frequency within the operating range of the lamp. This circuit includes a normally non-conducting switching device controlled by signals applied to two different control electrodes associated therewith. Coupled to the switching device is a timing circuit which charges to a predetermined value in response to current flow through the first switching device. When the synchronizing signals are applied to the first control electrode of the first device, the current flow through it charges up the timing circuit to the predetermined value whereupon it discharges. Means are provided for coupling a signal generated by this discharge to the second control electrode thereby cutting the first switching device off. The output frequency subdivided signal is then applied through switching means to operate the circuit which produces the voltage necessary to trigger the lamp. These output signals may be delayed to any desired point in the rotary cycle of the device under study. When a photograph is to be taken of the device under study, a special high intensity circuit is actuated whose function is to prevent the frequency-subdivided pulses from being applied to activate the trigger circuit until such time as a very high charging capacitor has been completely switched into circuit with the lamp. The first frequency-divided pulse received by the high intensity circuit after the end of the timing cycle starts the operation ofa second timing cycle whose beginning causes the production of a pulse which operates to fire the strobe lamp at a very high light intensity.

BRIEF DESCRIPTIONS OF DRAWINGS FIG. 1 is a block and schematic view of an overall stroboscope control system which embodies the invention.

FIG. 2 is a schematic view of one of the components shown in block form in FIG. 1.

FIG. 3 is a schematic view of another of the components shown in block form in FIG. I.

FIG. 4 is a schematic view of still another of the components shown in the block diagram of FIG. 1.

FIG. 5 is a schematic view of another of the components shown in the block diagram of FIG. 1.

FIG. 6 is a schematic diagram corresponding to one of the blocks shown in FIG. 1.

FIG. 7 is a schematic diagram of one of the blocks shown in FIG. 1.

FIG. 8 is a series of waveforms illustrating the operation of the system shown in FIGS. 1, 3 and 6, the waveforms being lettered to correspond to similarly lettered parts of FIGS. 3 and 6.

OVERALL SYSTEM FIG. 1

Referring to FIG. 1 there is shown an overall stroboscope control system which includes a power supply 10 that is connected to any appropriate line voltage source, for example. The power supply may be conventional and supplies the various other components of the system with appropriate operating voltages. It supplies in one mode of operation, for example, the current for a low intensity charging circuit 11 which is connected to one electrode of a stroboscope lamp such as the General Electric Type H-l5l lamp. In another mode, the power supply 10 is connected through a high intensity charging circuit 12 to the same electrode of the lamp 14. The low intensity charging circuit 11 is used when it is desired to illuminate the lamp 14 at a predetermined flashing rate in synchronism with rotation of the device under study by the stroboscope. The high intensity charging circuit 12 is switched in series with the lamp 14 when, instead of producing a multiplicity of flashes of light, it is desired to take a single photograph of the rotating device at a very high intensity of light.

An ionizing voltage circuit 13 is also provided which is connected to the trigger winding 15 associated with the lamp and, when the winding is energized at the desired time, fires the lamp. The ionizing circuit 13 may be entirely conventional. Its function is to step-up the relatively low level voltage appearing at its input from switch 19 to a much higher amplitude, say on the order of 10,000 volts, for firing the strobe lamp 14. It accomplishes this, for example, by controlling the discharge of a charging capacitor connected to power supply 10 through the primary of an output transformer having a very high secondary-to-primary turns ratio.

There is also provided a frequency dividing circuit 16 whose input may be connected to a source of external synchronizing signals associated with the rotating device under study. When the frequency of the external pulses is much higher than the rate at which the lamp 14 can flash, it is the function of the frequency divider 16 to supply pulses to the ionizing voltage circuit 13 at a sub-multiple of the frequency or repetition rate of the external sync signals. Unless the frequency of these input synchronizing pulses (which are generated in response to rotation of the device under study) is converted to a synchronous lower one, the stroboscope lamp 14 will either stay on continuously or not flash at all. Unlike prior stroboscope control circuits, the circuit 16 need not be manually set to a predetermined division ratio depending upon the frequency of the incoming sync pulses but rather accomplishes this automatically.

The output of the frequency dividing circuit 16 may be applied to a delay circuit 18 through a switch 17 and thence, through switch 19, to trigger the ionizing circuit 13. Alternatively, the output may be applied directly through switches 17 and 19 to the ionizing circuit. The delay circuit route is used when it is desired to shift the time of occurrence of the stroboscope flash to a desired point in selected cycles of rotation of the device under study. The delay circuit is bypassed when the frequency-divided sync pulses are to be applied directly to trigger the ionizing circuit 13.

Under certain circumstances, it may be desired to take a photograph of the device under study by means of a single, high intensity light flash. This is accomplished by using the high intensity timing circuit 20 when the camera shutter generates the desired signal. Immediately thereafter, the high intensity circuit 20 disables the switch 19 from transferring signals from either the delay circuit 18 or the dividing circuit 16 to the ionizing circuit. This is done to prevent harm to the contacts of the high intensity charging circuit due to a very high current surge that might be caused by the application of a trigger pulse to the lamp 14 just when switch contacts in the circuit 12 are closing.

FREQUENCY DIVIDING CIRCUIT 16 FIG. 2

The external synchronizing signals may be derived in any conventional manner from the operation of the object under test. For example, they could be generated by a shutter attached to the moving part which interrupts the path of light from a light source to a photoelectric cell, or they could be generated by a set of contacts which open and close once for each revolution of a rotating shaft, etc. These external sync signals, which can be sinusoidal or pulselike, for example, are applied by a coupling capacitor to the anode of diode 26 which passes only positive pulses. Capacitor 25 and resistor 28 cooperate to shape the incoming sync signals somewhat. The sync signals are then applied to the C gate electrode 27c of a silicon controlled switch indicated generally at the numeral 27. The A electrode 27a is connected through a voltage-dropping resistor 29 to a source of 8+ voltage which may be the power supply 10, for example. A secondary winding 30b of a transformer indicated generally at the numeral 30 is connected to the A gate electrode 27b and to one end of the resistor 29. A load resistor 32 is positioned between the C electrode and ground. The resistor 31 serves to establish a predetermined potential difference between the cathode gate 27c and the cathode 27d.

The silicon control switch, as is well known, can be turned on by a pulse of the proper polarity applied to the cathode gate electrode 270 and turned off by a pulse of the same polarity applied to the anode gate electrode 27b. The first positive sync pulse will turn on switch 27 so that current will flow from B+ through the switch to ground producing a voltage drop across the resistor 32. The value of the load resistor 32 is chosen to allow enough current to flow through the switch to keep it in the conductive state. As a result of the flow through the switch 27, a positive pulse will be propagated through the capacitor 34 and across the output resistor 35 to the switch 17. When the switch 27 is rendered conductive, current will also flow into a timing circuit which comprises the unijunction transistor 36, the resistor 37 and the timing capacitor 39. The value of resistor 37 and of the capacitor 39 determine the maximum output frequency of the frequency dividing circuit. When sufficient current flows through the resistor 37 into the capacitor 39 to charge the latter to a predetermined level, the unijunction transistor 36, which is normally non-conductive, will turn on and the capacitor will discharge through it and the transformer primary winding 30:: to ground. The timing circuit is isolated from the cathode of switch 27 by theresistor 40. The windings 30a and 30b are mutually coupled, but the winding 30b is arranged to produce an induced voltage of the same polarity. This induced voltage will turn off the switch 27. Thus, the first positive sync pulse will start the flow of current through switch 27 but from that time on to the end of the timing cycle the circuit 16 operates independently of the receipt of intervening external sync signals. The switch 27 stays on only as long as the time constant of the timing circuit permits. Consequently, the very high repetition rate of the sync pulses from the object under study is effectively reduced to a rate which is within the operating (flashing) range of the strobe lamp.

SWITCH 19 If the switch 17 is in the delay-off condition, the

frequency-divided pulses from circuit 16 will be bypassed around the delay circuit 18 and applied to a switch 19 (FIG. 3). Switch 19 includes sets of contacts 43 and 44 which are normally closed and 45 and 46 which are normally open. These sets of contacts are operated by energization and de-energization of the winding 42 (FIG. 6) as will be explained below. In addition, there is a voltage-dropping resistor 47 connected between ground and one of the contacts of the set 45. With the switch 17 in the delay-off" position, therefore, the frequency-divided pulses will be applied via switch contacts 44 and 43 to actuate the ionizing voltage circuit 13 which supplies the trigger coil 15 with the trigger voltage.

DELAY CIRCUIT 18 FIG. 5

When the flashing of the strobe lamp 14 does not occur at the desired times in the rotation cycles of the object under test, the delay circuit 18 can be used to vary the phase of the frequency-divided signals from circuit 16 before they are applied to the ionizing voltage circuit 13. With the switch 17 in the delay on position, the positive-going output pulses of circuit 16 developed across load resistor 35 are applied via diode to the C-gate electrode 560 of silicon control switch 56. The switch 56 is so biased that this positive pulse will turn it on so that current will flow from B+ through voltage dropping resistor 57 and laod resistor 58 to ground. The current through the switch 56 is sufficient to keep it on and the voltage developed across resistor 58 will be applied to a timing circuit comprising unijunction transistor 60, resistor 61, variable resistor 62, and timing capacitor 63. The voltage on the capacitor 63 will build up at a rate and to an extent determined by the RC constant of the resistor 61 and the setting of resistor 62, and the capacitor 63. At the end of the timing cycle, the capacitor 63 discharges through the transistor and the primary winding 65a of the transformer 65 to ground. Since the winding 65a is mutually coupled to the secondary winding 65b, there will be a positive pulse induced in the winding 65b and applied lamp. Within limits the variable resistor 62 may be adjusted to produce output pulses delayed by a time not exceeding the inter-pulse time of the frequency-divider pulses.

SWITCH 19 FIG. 3

The frequency-divided pulses may be applied directly to the switch 19 or may be delayed in circuit 18 and then applied to the switch 19. The switch 19 serves to distribute the delayed or undelayed frequency-divided sync pulses to the ionizing circuit 13. As will be explained later, when a photograph is to be taken, it also cooperates with the high intensity circuit 20 to prevent the application of any triggering pulses to the coil in the interval between complete inactivation of the low intensity circuit 11 and complete activation of high intensity circuit 12. This is done to protect switch contacts in those respective circuits from the effects of unduly large current surges in that interval.

Whenever the high intensity circuit is not on, the sets of contacts 43 and 44 are closed. This enables the frequency-divided pulses, delayed or not, to be applied to the ionizing circuit for triggering the lamp 14. At the same time the sets of contacts 45 and 46 are open so that no pulse from the high intensity circuit can be applied to circuit 13. The states of the sets of contacts in the switch 19 (and of the contacts 51 and 53 in circuits 11 and 12 respectively) are changed by energization of coil 42 in the circuit 20, as will be explained later, when the circuit 20 is brought into service for taking a single photograph of the illuminated object under test. Resistor 47 is used to develop a voltage pulse when the contacts 45 and 46 are closed.

LOW INTENSITY CHARGING CIRCUIT FIG. 4

Normally, this circuit is operating to supply the voltage across lamp 14 necessary to enable its being flashed repeatedly when the coil 15 is energized in response to the sync pulses. The voltage produced across lamp 14 is sufficient to produce low intensity flashes adequate for visual observation. The circuit 11 is supplied with a voltage from the power supply through choke coil 49 and normally closed contacts 51. This voltage charges the relatively small charging capacitor 50 so that voltage builds up across this capacitor up to a predetermined level whereupon it discharges through the lamp 14. The current passing through the lamp 14 is of relatively low intensity, but is entirely adequate for repetitive flashing of the object under study at frequencies up to 200 cps, for example, depending upon the values of the circuits components. When, however, a photograph is to be taken, the contacts 51 are opened in response to energization of coil 42 and the contacts 53 of the high intensity timing circuit 20 are simultaneously closed.

HIGH INTENSITY TIMING CIRCUIT 20 FIG. 6

It is often desired to obtain a photograph of the object under study by the strobe illumination. To do this, the circuit 20 is provided which can be activated at any time without previously disabling the frequency dividing circuit 16 or the delay circuit 18. However, because of the damage that might be done to the set of contacts 53 in the circuit 12 if the ionizing circuit 13 is actuated by a sync pulse before contacts 53 are fully closed, the circuit 20 must have special features. The circuit 20 cooperates with the switch 19 and with the contacts 51 and 53 to insure that no sync pulses are applied to the ionizing circuit for triggering the coil 15 until such time as the set of contacts 53 have been com pletely closed. Were this not the case, since the charging capacitor 52 of the high intensity charging circuit has such a large value, the voltage across the contacts just before the make" of contacts 53 would produce a very high current arc between these contacts and weld them together or otherwise destroy them.

Accordingly, whenever a photograph is to be made, the actuation of the shutter of the camera synchronously closes the switch 70. Thereupon the voltage at point A goes to ground potential as shown in FIG. 8, Part A and current from B+ passes through the switchactuating coil 42 to ground. Energization of coil 42 starts the closure of contacts 45, 46 and 53 but since the switch 19 is an electro-mechanical device a finite period of time, say, 10-20 milliseconds, will elapse before complete closure is effected. The switch contacts 45, 46 and 53, as well as the contacts 43, 44 and 51 respectively open and begin to close simultaneously. The switch or relay may be of the type having three movable arms, each with contacts mounted on both sides at the end, which move between two sets of contacts, each set having three contacts. The broken line 42a indicates the magnetic coupling of the coil 42 to the various contacts controlled by it.

Ordinarily, the transistor 71 is conducting but the current through the switch effectively shorts out resistors 74 and 86 and cuts off the transistor. Consequently, a pulse is generated from B+ (FIG. 8, Part B) which goes through the resistor 88, coupling capacitor 72 and diode 73 to switch 75. This positive pulse can not yet be transferred to the ionizing circuit through capacitors 81 and 83 because the switch contacts 45 and 46 are now open. This pulse is applied to the cathode gate of silicon control switch 75 after being delayed somewhat by the capacitor 72 and resistor 77.

Upon application of this pulse to the switch 75, the latter begins to conduct to ground through load resistor 78 (FIG. 8, Parts C and D). A charging voltage is produced that is applied to the resistor 82 and the capacitor 84 which constitute the RC portion of the timing sub-circuit. Resistor isolates the timing circuit from the cathode of switch 75. While the timing circuit is charging up, the contacts 45 and 46 finally close (FIG. 8, broken line sc). However, since the switch 75 is still conducting, the positive pulses at F still do not produce corresponding pulses at G (see FIG. 3) since there is a common power supply 10 for the circuit 20 and for the frequency-dividing and/or delay circuits 16 and 18, whichever is the source of pulses at G.

When the voltage finally builds up to a predetermined point determined by the RC constant, on the order of say, 50 milliseconds, the capacitor 84 will discharge through the unijunction transistor 85 and the winding 87a to ground. The passage of this current (FIG. 8, Part E) through winding 87a will induce a positive pulse in the coil 87b which is connected between anode and anode gate of the switch 75, thereby turning the latter off and completing the timing cycle (FIG. 8, Part C D, broken line t-"l Thus, any pulses which have appeared at the input to switch 19 (FIG. 8, Part F) between the time that the flash synchronizing switch 70 closes and the end (Fl of the timing cycle cannot be transferred to the ionizing circuit. Consequently, there is no danger to the contacts 4S and 46 due to the arrival ofa pulse at the input to the ionizing circuit just as contacts 53 are about to close. As soon as the contacts 45, 46 and 53 have been closed, the next positive pulse 95 (FIG. 8, Part F) from the delay circuit or from the frequency-dividing circuit will be transmitted via closed contacts 46 through the capacitor 83 and the diode 73 to the cathode gate of switch 75 thereby causing it to conduct and start another timing cycle 96 (FIG. 8, Part C+D). The increase in the positive potential at D is transmitted via capacitor 81 and newly closed contacts 45 to ionizing circuit 13 to trigger lamp 14 on" for a high intensity flash since capacitor 50 is then in circuit with the lamp.

HIGH INTENSITY CHARGING CIRCUIT 12 FIGURE 7 Whenever a coil 42 is energized upon closure of the switch 70, the switch contacts 53 will close and contacts 51 will open so that only the capacitor 52 is in circuit between the power supply and the lamp 14. The value of the capacitor 52 is much greater than that of the capacitor 50. For example, it may be on the order of 125 times as large. Consequently, a much higher voltage will be built up across it, which, when the contacts 53 are closed, will produce a much greater current to the phototube resulting in a much higher light intensity for photographic purposes. The circuit 20 is provided to prevent damage to contacts 53 when they just are about to close due to the large current that might be drawn should the lamp l4 fire at this time.

GENERAL REMARKS While the circuit components of the present invention may have different values depending upon the flash lamp used, or on the input synchronizing signal frequency, or on the amount of delay desired, or the characteristics of the associated camera, one set of component values which has proved to be entirely satisfactory is set forth below:

Component No. Value 25 0.0022 mfd 26 IN207I 28 10 K ohms 29 12 ohms 31 l K ohm 32 6.8 K ohms 34 0.0022 mfd 35 ll( ohm 37 5.] K ohms 39 l mfd 40 390 ohms 49 4-15 henries 50 4 mfd 52 l0 mfd 61 2.2K ohms 62 50K ohm pot 63 0.4 mfd 64 390 ohms 7] 2N3707 72 0.047 mfd 73 lN207l 76 115 ohms 77 K ohms 79 6.8K ohms 80 470 ohms 81 0.047 mfd 82 2.2 K ohms 83 0047 mid 84 0.22 mfd 85 2Nl671 88 10K ohms 89 12 ohms 90 390 ohms It should also be understood that the system may be used with internally generated synchronizing signals rather than those generated by couplings to the device under test. This would require certain modifications of the circuit but this does not affect the operation of the present invention in any material way.

While the invention and its various novel subassemblies and sub-circuits have been described with reference to a stroboscope system it is evident that the overall system and the novel sub-circuits could find utility in other applications. For example, the frequency division circuit has broad utility as does the delay circuit. Furthermore, the high intensity timing circuit conceivably could be used in other applications where it is desired to produce a switching or delaying action for signals.

Still other variations and applications will occur to one skilled in the art upon perusal of this specification and the drawings herein which do not depart from the essence of the invention. Consequently, it is desired that this invention be limited solely by the claims herein:

I claim:

1. A switching circuit including the combination of pulse generating means for generating a reference pulse in response to actuation of a switch,

a timing circuit coupled to said pulse generating means, said timing circuit being effective to produce a first timing cycle in response to said reference pulse and to produce a first timing pulse corresponding to said first timing cycle,

signal means for applying signals to said timing circuit during and after said first timing cycle, said signals applied to said timing circuit during the first timing cycle having no effect upon said circuit, the first signal applied to said timing circuit after said first cycle being effective to initiate a second timing cycle and production of a second timing pulse,

coupling means for applying at least a portion of said second timing pulse to a utilization circuit, and

last means interconnected with said coupling means and responsive to actuation of said switch for changing the condition of a second switch associated with said utilization circuit.

2. The switching circuit of claim 1 wherein said last means includes a first inductance coupled to said pulse generating means.

3. The switching circuit of claim 2 wherein said pulse generating means includes a first semiconductive device which is normally biased conductive but is rendered nonconducting by closure of said switch.

4. The switching circuit of claim 3 wherein said timing circuit includes a normally nonconductive second semiconductive device having at least first and second control electrodes,

means coupling said first control electrode to said first semiconductive device for receiving said pulses, said second semiconductive device being biased conductive in response to said pulses, and

a third semiconductive device and a charging circuit in said timing circuit and coupled to said second semiconductive device,

said charging circuit discharging through said third semiconductive device after a predetermined time depending on the time constant of said charging circuit, said discharge of said third device cutting off said second device.

5. The switching circuit of claim 4 including a second inductance in said timing circuit coupled to said second control electrode of said second semiconductive device,

a third inductance in said timing circuit coupled to said third semiconductive device in said discharge path, said second and third inductances being mutually coupled for the production of an induced voltage in said second inductance in response to the discharge of said charging circuit through said third inductance, said induced voltage cutting off said second semiconductive device.

6. The switching circuit of claim 2 including third switch means coupled to said timing circuit and to said utilize means, said third switch means being actuated by current through said first inductance when said first switch is closed for applying a portion of said second timing pulse to said utilizing Clfcuit.

7. The switching circuit of claim 5 wherein said first semiconductive device is a transistor,

said second semiconductive device is a silicon controlled switch whose first and second electrodes are cathode and anode gates, respectively,

said third semiconductive device is a uni-junction transistor, and

said charging circuit includes an RC circuit coupled to said uni-junction transistor for discharge therethrough and through said third inductance. 

1. A switching circuit including the combination of pulse generating means for generating a reference pulse in response to actuation of a switch, a timing circuit coupled to said pulse generating means, said timing circuit being effective to produce a first timing cycle in response to said reference pulse and to produce a first timing pulse corresponding to said first timing cycle, signal means for applying signals to said timing circuit during and after said first timing cycle, said signals applied to said timing circuit during the first timing cycle having no effect upon said circuit, the first signal applied to said timing circuit after said first cycle being effective to initiate a second timing cycle and production of a second timing pulse, coupling means for applying at least a portion of said second timing pulse to a utilization circuit, and last means interconnected with said coupling means and responsive to actuation of said switch for changing the condition of a second switch associated with said utilization circuit.
 2. The switching circuit of claim 1 wherein said last means includes a first inductance coupled to said pulse generating means.
 3. The switching circuit of claim 2 wherein said pulse generating means includes a first semiconductive device which is normally biased conductive but is rendered nonconducting by closure of said switch.
 4. The switching circuit of claim 3 wherein said timing circuit includes a normally nonconductive second semiconductive device having at least first and second control electrodes, means coupling said first control electrode to said first semiconductive device for receiving said pulses, said second semiconductive device being biased conductive in response to said pulses, and a third semiconductive device and a charging circuit in said timing circuit and coupled to said second semiconductive device, said charging circuit discharging through said third semiconductive device after a predetermined time depending on the time constant of said charging circuit, said discharge oF said third device cutting off said second device.
 5. The switching circuit of claim 4 including a second inductance in said timing circuit coupled to said second control electrode of said second semiconductive device, a third inductance in said timing circuit coupled to said third semiconductive device in said discharge path, said second and third inductances being mutually coupled for the production of an induced voltage in said second inductance in response to the discharge of said charging circuit through said third inductance, said induced voltage cutting off said second semiconductive device.
 6. The switching circuit of claim 2 including third switch means coupled to said timing circuit and to said utilize means, said third switch means being actuated by current through said first inductance when said first switch is closed for applying a portion of said second timing pulse to said utilizing circuit.
 6. The switching circuit of claim 2 including
 7. The switching circuit of claim 5 wherein said first semiconductive device is a transistor, said second semiconductive device is a silicon controlled switch whose first and second electrodes are cathode and anode gates, respectively, said third semiconductive device is a uni-junction transistor, and said charging circuit includes an RC circuit coupled to said uni-junction transistor for discharge therethrough and through said third inductance. 